1. Field of Invention
This invention relates to a sintered powder metal plain bearing having the pores thereof infiltrated with an anti-galling metal, and more particularly to such a bearing having a relatively thin wall powder metal component which is secured to and supported by a solid metal backing or shell.
2. Description of the Prior Art
U.S. Pat. Nos. 4,105,263 and 4,207,658, both of common assignee with the present invention, show a sintered powdered-metal bearing for a rock bit and a method of making it, respectively, wherein a metal alloy powder is compressed into the desired cylindrical shape for a journal bearing, providing a porous matrix approximately 85% dense, with the pores thereof being substantially in communication throughout. After pressing and sintering such cylindrical member, the pores are infiltrated with an anti-galling material such as silver-manganese alloy to provide a premium rock bit journal bearing having, at the bearing surface, alternating areas of alloy steel and silver-manganese providing strength and lubricity for the bearing surface.
It is thus apparent that the silver-manganese anti-galling infiltrating material comprises, by volume, approximately 15% of the powdered-metal bearing structure but is effective for lubricity only at the bearing surface. In that this material is relatively expensive because of its silver content, it has been determined that a substantial cost reduction could be obtained by limiting the wall thickness of the bearing. However, it is accepted practice in powder metallurgy compaction that, in order to produce uniformly dense powder metallurgy parts, even when using a double acting press, the configuration of pressed powder metal parts is to have a height (in this instance, the axial dimension) no greater than a factor of four times the wall thickness. Otherwise, it has been found that the central portion of the height of the pressed part was less dense than either end portion, giving the part a non-uniform strength, generally unsuitable for high load and stress conditions such as encountered in rock bit bearing applications.
U.S. Pat. No. 4,172,395, likewise of common assignee, shows a technique and structure for making a relatively thin walled sintered powdered-metal bearing for a rock bit wherein the powdered-metal bushing is formed and compacted in situ in a recess in the bearing cavity of the cone cutter and, during sintering, adheres to the base metal of the cone. The pores thereof are subsequently filled with an anti-galling material and the bearing is then hardened in accordance with the typical treatment for finishing the sintered bushing. Such method has never been commercially acceptable because of the manufacturing problems for such in situ formation. Further, it would require compaction with a single action press resulting in the axially innermost half of a bushing of the necessary axial extent (i.e. height), being considerably less dense than the outer half, again resulting in non-uniform strength and likely early failure.
U.S. Pat. No. 4,402,617, again of common assignee, discloses yet another thin walled sintered powdered metal bushing for a rock bit wherein the bushing is formed and infiltrated prior to it being seated in interfering engagement within a bearing shell and wherein the structure on the bearing shell in cooperation with structure in the bearing cavity of the cone captures the bushing so that it cannot be displaced during the extreme forces encountered in a rock bit. As contemplated by this invention, although not stated explicitly, the powdered-metal bushing was first compacted with a wall thickness consistent with powder metallurgy technology (i.e. a height to depth ratio less than 4) to obtain consistent uniform strength throughout, then after sintering, filled with the anti-galling metal to provide a machinable structure, and then machined to the thin-walled configuration shown, capturing for recycling, the infiltrant of the machined-away material.
Journal bearings for a rock bit made according to the latter patent have not been commercially available because of either the above manufacturing complexity and expense and the potential for assembly errors (i.e. it is apparent from FIG. 2 of this patent that the assembly of the bushing could be reversed such that such thin wall bushing could be positioned adjacent the O-ring and thereby not axially retained).
It is well known that the silver-manganese alloy which has been commercially used as the anti-galling metal infiltrated into the powder metal sintered bushing described in the above patents was initially developed as a brazing compound exhibiting, in its molten condition, a great affinity to flow between closely fitting adjacent faces by capillary action and also forming nominal fillets yet having sufficient flow resistance to prevent flow completely out of the brazed joint. Because of these characteristics and the inherent premium anti-galling properties of the silver it came to be used, as is made apparent in the identified patents, as an infiltrant for the powdered-metal bushings therein discussed. However, in this regard, it is also known that when the silver-manganese is infiltrated into the porous powdered metal base material from one surface of said material, continued heating of the base material and the silver-manganese until the base material is completely filled, causes the silver-manganese adjacent the surface from which the infiltration occurred to segregate into a predominantly manganese layer. This high manganese layer, when subject to subsequent heat treatment, as is required in finishing the base material for use as a bushing, results in the corresponding layer of the base material being converted to a stable Austenite phase which is inherently soft and not responsive to thermal treatment. Thus, to accommodate this phenomenon, additional base material or stock was required for such layer, which was then subsequently machined away to expose a lower portion of the base prior to further thermal treatment of the bushing.